A typical range extender is provided as an integral part of a hybrid electric vehicle to improve its travel range. For example, U.S. Pat. No. 5,264,764 discloses a hybrid electric vehicle comprising an inverter/motor electric drive system, a vehicle controller, a range extender and a traction battery. The inverter/motor electric drive system is composed of an electric motor and an inverter/motor control unit. In response to a hand or foot actuated speed control actuator, a vehicle controller applies a torque command signal to the electric motor to drive the vehicle wheels through a transmission. The range extender, which includes an internal combustion engine and an alternator, is actuated by the vehicle controller to provide electric power to the inverter/motor drive system and/or the traction battery. The internal combustion engine rotationally drives the alternator to output electric power, which is applied to recharge the traction battery or to energize the motor. The inverter/motor control unit is responsive to the torque command signal to provide sufficient electric power from the traction battery and/or the range extender to the electric motor to drive the vehicle at a desired speed.
Another example of a range extender incorporated in a hybrid electric vehicle is disclosed in U.S. Pat. Nos. 6,622,804 and 6,333,620, which describe a series type hybrid electric vehicle including electric motors powered by a battery array, a vehicle control unit, an internal combustion engine and a generator driven by the internal combustion engine for charging the battery array. The internal combustion engine is controlled by the vehicle control unit to run continuously during operation of the vehicle. The charging of the battery array by the generator is controlled by the vehicle control unit depending on a driving mode of the vehicle, e.g., in accordance with power output, power consumption and rpm of the electric motors. The power generated by the generator is generally maintained constant but may be decreased if the state-of-charge of the battery array approaches an upper control limit at which the battery array may be overcharged, and may be increased if the state-of-charge approaches a lower control limit at which the battery array would be unable to drive the electric motors with enough torque to propel the vehicle appropriately.
The above-described systems are typical of a range extender that is designed specifically for the hybrid electric vehicle in which the range extender is installed. The range extender is controlled by the vehicle controller to interact with the electric motor of the vehicle. In a hybrid vehicle, the electric motor is used primarily for low-speed cruising or to provide extra power for acceleration or hill climbing. When braking or coasting to a stop, the hybrid vehicle uses its electric motor as a generator to produce electricity, which is then stored in its battery pack. Thus, the electric motor in hybrid vehicles improves fuel economy compared to a vehicle equipped only with an internal combustion engine. Unlike all-electric vehicles, which have a limited travel range and create inconvenience for users by requiring frequent recharging at charging sites, hybrid vehicles do not need to be plugged into an external source of electricity to charge batteries.
However, hybrid vehicles present challenges to be addressed in order to be suitable for widespread implementation. For economic feasibility of the hybrid vehicle, which can be more expensive than a non-hybrid vehicle, cost should be minimized and advantages should be attained from other vehicle operational considerations. Emissions caused by the internal combustion engine should be reduced or eliminated if possible. The weight and size considerations involved with the use of both electric motor propulsion and an internal combustion engine with its fuel supply are important factors in vehicles such as compact cars and trucks.
Hybrid vehicles and all-electric vehicles each have their advantages and disadvantages. Each type of vehicle obtains benefits from the provision of a range extender. The need exists for a portable range extender that can be electrically connected to an all-electric vehicle to enable the vehicle to travel longer distances, if necessary. For shorter travel distances that do not require range extender functionality, it would be desirable to avoid the weight of, and space occupied by, the unused range extender. The portability of a range extender would make it amenable to support vehicles of different types and be movable readily from one vehicle to another. Thus, it would be desirable that such portable range extender be operable independently of the controller of the vehicle. Improvements are also needed to reduce emissions and increase fuel efficiency.